It is well known that a method for directly observing the surface of a joint by inserting an arthroscope into the cavitas articulare, a method of intuitively diagnosing a degree of injury and hardness of the surface of a cartilage by contacting a probe with a damaged area of an articulara, a method for observing images picked up by the MRI, and so forth. With direct-vision observation by use of the arthroscope or the probe, however, none other than a surface state can be discriminated and it has been impossible to discriminate a crack present inside an articular cartilage. Similarly, it has been impossible to quantitatively determine the mechanics•structural characteristics of a subchondral bone area. Meanwhile, with the MRI method, it is impossible to observe a slight changes occurring in the cartilage. Further, it is difficult to make a diagnosis at a spotted area.
It is difficult to evaluate numerically the mechanics•structural characteristics of an articular cartilage by the conventional diagnostic methods, because the diagnosis criteria are not clear and the results are different depending on the observer. Further, in the numerical evaluation, the results are expressed with numbers, however, the operator might misread the numbers on a display, since the operator is engaged with several tasks simultaneously during a surgery operation.
Bones and articular cartilages have important roles for motions of the body and for supporting a body weight, therefore it is preferable that the bones are hard (high Young's modulus) and, in case of the articular cartilages, it is preferable that the hardness falls within a certain range because the articular cartilages contributes not only to the support of the body weight, but also to absorbing shock.
Similarly, blood vessels should have adequate hardness in order to fulfill its function for flowing pulsating blood because the blood vessels are susceptible to damage if they are excessively hard and unable to withstand blood pressure if excessively soft.
Ultrasonic signals reflect at a boundary of tissues having different acoustic impedances and it is possible to display a respective image of internal organs and tissues by utilizing the echo signals from the boundary. An acoustic velocity correlates with Young's modulus, which is an index of hardness, and when in measuring a substantially constant density material, the echo signals contain mechanical characteristics of the materials (See Non-patent document 1, and Non-patent document 2).
As shown in FIG. 1, a load-bearing part of a knee joint cartilage, which is likely susceptible to articular diseases, is moved to an area in the vicinity of the body surface when the knee is bent. The tissues of this area constitute a substantial laminar structure consisting of skin 4, articular cartilage 30 (several mm in thickness) and subchondral bone 31.
The skin 4 and the articular cartilage 30, among these tissues, containing rather much water, and have almost the same densities and relatively the same acoustic impedance although they have slightly different acoustic velocities. The subchondral bone 31 has larger values in both the acoustic velocity and density compared with those of the skin 4 and the articular cartilage 30, and has a significantly different acoustic impedance from both the skin 4 and the articular cartilage 30.    Patent document 1 JP 10-118062 A    Patent document 2 JP 2002-136520 A    Patent document 3 JP 11-316215 A    Patent document 4 JP S61 (1986)-290942 A    Patent document 5 JP 2002-345821A    Non-patent document 1 Ultrasonic Handbook by Ultrasonic Handbook Compilation Committee, published by Maruzen    Non-patent document 2 “Ultrasonic Waves and Material” by Japan Material Science Society, published by Shokabo    Non-patent document 3 “Review on Ultrasonic Evaluation by the Double Probe Technique (Ultrasonic Evaluation from Outside the body)” by Okamoto, Mori, et al., Japan Machinery Society, Proceedings, 2006 annual general meeting, Vol. 15, pp. 153-154, 2006